It is known in the art to provide for phase and speed control of propellers on multiple engine aircraft as is shown in U.S. Pat. Nos. 4,659,283 and 4,653,981.
Propeller synchronizers have been used on multi-engine propeller driven aircraft to maintain a selected phase angle relationship between the master propeller and each of the designated slave propellers. Once the propellers are synchronized, the aircraft fuselage experiences minimum acoustic noise and vibration. The propeller synchronizer device provides a relative phase angle for a given slave propeller by changing the slave propeller speed.
In order to minimize noise in propeller driven aircraft, it is important to maintain a desired phase relationship among all aircraft propellers. This disclosure describes an improved method for controlling the phase relationship between two such propellers.
Present-day propeller synchronizers typically utilize an electrical pulse to detect the time that each propeller is at a particular blade position. For a given propeller, the time between successive pulses may be used to determine propeller speed. Using time as a measure of propeller blade position, the phase relation between any two propellers can be determined by the length of time between an electrical pulse from one propeller and that from the other propeller.
In the simplest implementation, the sensed phase .DELTA..phi. between the slave propeller and the master propeller is derived by comparing one particular blade of the slave propeller versus one particular blade of the master propeller. FIG. 1 shows .DELTA..phi. versus time for the case where the speed of the slave propeller (N.sub.PS) exceeds the speed of the master propeller (N.sub.PM) by 1 RPM. Note that 1 RPM is equivalent to 6 deg/sec and that the range of .DELTA..phi. is .+-.180.degree. for this case.
In a more sophisticated implementation, the sensed phase .DELTA..phi. between the slave propeller and the master propeller could be derived by comparing any one of the blades of the slave propeller versus one particular blade of the master. The obvious advantage is that at any given time, the blade of the slave propeller which had its position nearest to that of the master, could be used to define .DELTA..phi.. If the number of blades per propeller were B, then the range of .DELTA..phi. would be .+-.(180.degree.). FIG. 2 shows .DELTA..phi. versus time for a four-bladed propeller (B=4) where N.sub.PS -N.sub.PM =1 RPM. The range of .DELTA..phi. for this example is .+-.45.degree..
It is also possible to select from a subset of blades which are equally spaced, to determine .DELTA..phi.. For a propeller with B blades, if L is a prime factor of B, then ##EQU1## represents all possible ranges for .DELTA..phi.. For example, for a propeller with 6 blades (B=6), then L=1, 2, 3 or 6 and possible ranges for .DELTA..phi. are .+-.180.degree., .+-.90.degree., .+-.60.degree. and .+-.30.degree..
Phase error (.phi..sub.E) is the angular difference between sensed phase .DELTA..phi. and a reference phase difference (.DELTA..phi..sub.REF). .DELTA..phi..sub.REF is the phase offset from perfect alignment of blades which is often necessary in providing the lowest noise synchronization of propellers. .DELTA..phi..sub.REF is determined by the characteristics of the particular aircraft, propellers and engines.
Present propeller synchronizers typically utilize one electrical pulse per propeller revolution so that sensed phase provides a phase error measurement up to .+-.180.degree. as described above. In addition, present propeller synchronizers some form of "start circuit" to assure that the master and slave propeller speeds are nearly the same and that the phase error is small so that the synchronizer control laws will avoid the condition where phase error has a numerical discontinuity. The description of the invention herein is based on the assumption that the range of .DELTA..phi. is .+-.180.degree..